Various methods have been proposed for the liquefaction of coal, including a dry distallation process in which coal is heated at high temperatures and the thus-distilled tar component is recovered, a solvent extraction process in which coal is extracted with a solvent, an extraction-chemical decomposition process (for example, EDS Process) in which decomposition of coal is achieved using a hydrogen-donating solvent simultaneously with its extraction, an extraction-hydrogenation process (for example, SRC Process) in which solvent extraction is performed while supplying hydrogen gas under high pressure, and a direct hydrogenation process (for example, H-Coal Process) in which hydrogenolysis of coal is performed in the presence of a catalystic while supplying hydrogen gas under high pressure.
The dry distillation process is not desirable because the liquefaction yield is low with respect to ordinary coal although the process is simplified. In the solvent extraction process, benzene, toluene, xylene, carbolic acid, cresol, methylnaphthalene, creosote oil, anthracene oil, etc., are used to achieve the solvent extraction of coal. This process is not desirable because the extraction efficiency, i.e., liquefaction yield, is poor and it is, therefore, necessary to lengthen the extraction time.
In accordance with the extraction-chemical decomposition process, a hydrogen-donating solvent, such as tetralin, a mixture of tetralin and cresol, hydrogenated creosote oil, and hydrogenated anthracene, is added to coal and heated at about 400.degree. to 480.degree. C. to achieve the extraction-decomposition of coal. This process is advantageous in that the liquefaction yield is good and the reaction time is short. In using a mixture of tetralin and cresol, however, a problem arises in that when the mixture is heated at 400.degree. to 480.degree. C., high pressure is developed because both tetralin and cresol have boiling points of about 200.degree. C. Also, in the case of hydrogenated creosote oil, hydrogenated anthracene oil, etc., high pressure is developed during the liquefaction reaction because their volatile component contents are high. Furthermore, in hydrogenating creosote oil or anthracene oil to prepare the hydrogenated creosote oil or anthracene oil, the hydrogenation reaction proceeds excessively and, therefore, it is difficult to perform the reaction continuously.
As is well known in the art, such hydrogenated oils have been produced batchwise using a device such as an autoclave. That is, a nickel-molybdenum catalyst is used in an amount of from 5 to 10% by weight based on the free oil, and hydrogenation is carried out at a temperature of from 380.degree. to 450.degree. C. and a pressure of from 100 to 200 kg/cm.sup.2 (gauge pressure) for a period of from 2 to 4 hours. However, hydrogenation using a continuous flow type fixed bed reactor under conditions comparable to the above-described batchwise reaction conditions is very difficult in view of liquid space velocity. Even when hydrogenation is carried out under similar conditions, i.e., the same reaction temperature and reaction pressure, a substantial amount of deposition of carbon occurs in the catalyst layer.
When the feedstock for the production of the coal liquefaction solvent of the invention (i.e., a hydrocarbon mixture from which an 80% by weight or more portion is distilled away then heated at a temperature of from 320.degree. to 550.degree. C.) is hydrogenated without the addition of phenol and/or alkylphenols, the hydrogenation reaction proceeds either excessively or insufficiently. This will be hereinafter explained in detail by reference to the hydrogenation of pyrene which is a tetracyclic aromatic hydrocarbon as one of the major components contained in the feedstock. In the hydrogenation of pyrene, it is considered that partially hydrogenated products, such as dihydro-, tetrahydro-, hexahydro-, octahydro-, and decahydro-pyrenes, and a completely hydrogenated product, i.e., perhydropyrene, are produced. It is, however, difficult to obtain efficiently hydrogenated products having a partial hydrogenation rate of less than 50%, i.e., dihydro-, tetrahydro-, and hexahydro-pyrenes, since the hydrogenation reaction successively proceeds, forming partially hydrogenated products having a partial hydrogenation rate of 50% or more, e.g., octahydro- and decahydro-pyrenes, perhydro-pyrene, and mixtures thereof. These partially hydrogenated products having a partial hydrogenation rate of 50% or more have poor hydrogen-donating properties and their coal-dissolving powers are also poor. Therefore, they are not suitable for use as solvents for the liquefaction of coal according to the extraction-chemical decomposition process.
In brief, in the hydrogenation of a hydrocarbon mixture from which an 80% by weight or more portion is distilled away on heating at a temperature of from 320.degree. to 550.degree. C., and which is to be used as a feed for the production of a solvent for coal liquefaction as described hereinafter, the amount of hydrogen consumed is large and the liquefaction efficiency of producing partially hydrogenated products is poor. This results in the formation of large amounts of partially hydrogenated products having a partial hydrogenation rate of 50% or more. In the coal liquefaction technology, therefore, it is very desirable from an economic standpoint to reduce or control the formation of such highly hydrogenated products. Many difficulties, however, are encountered in attaining this object by conventional methods.
Both the extraction-hydrogenation process and the direct hydrogenation process are disadvantageous in that a large amount of hydrogen is needed. Particularly, in the case of the latter process, a catalyst which can be used over a long period of time has not yet been developed. Thus, many problems arise in the commercial practice of the processes.